Question 1
This doesn't make much sense other than as an academic question. I would venture to say that the server will act as a router to the Internet, i.e., one NIC connected to the network and another NIC connected to whatever device (usually a modem) is available to connect to the Internet.
Question 2
The clients should be configured to use DHCP (though you could also visit all clients if you prefer and use static IP addresses, define the default gateway, etc.).
You setup a DHCP service on the server and configure the scope properly. At a minimum, you will provide the following info:
- Address Pool (range of local IP addresses that can be assigned to clients)
- Scope Options
- Router (the server itself)
- DNS Server (in this case it probably would be the ISP's DNS or any DNS that you prefer)
Question 3
Do some reading on it:
http://en.wikipedia.org/wiki/Active_Directory
Actually you can also read info on DHCP, etc.